The invention relates generally to metal oxide non-linear resistors known as varistors, and more particularly to compositions for forming varistor blocks containing mostly zinc oxide.
One important use of varistors is in overvoltage protection devices for power transmission and distribution lines. Connected between a high voltage line and ground, the varistors are essentially non-conducting below predetermined breakover voltage level. When the breakover voltage of the varistor is exceeded, there is a sharp rise in current through the device. Ideally, after breakover a varistor will pass current to the extent necessary to prevent any further increase in voltage. The non-linear voltage-dependent resistance of varistors enables them to effectively protect against overvoltages on power lines caused by lightning bolts or the like.
Varistors comprise a sintered body having metal contacts on opposed faces. Operating parameters are measured in several ways. One measure of the breakover voltage of a varistor is termed the one milliamp voltage (1 mA V), which is the voltage per centimeter of thickness when the device is conducting current at a density of 1 mA per square centimeter. Device linearity is evaluated by determining the changes in voltage at increasing current levels. One measure of linearity is termed the voltage ratio, which is the voltage per centimeter of thickness across the device at a current density of 75 amps per square centimeter, divided by the 1 mA voltage. Another measure of varistor performance is surge durability, which is determined by the decrease in the 1 mA voltage after a large current (for example, 500 amps per square centimeter) has been passed through the device. Surge durability measures the resistance of the device to current channeling in the varistor body under severe loading conditions.
Various compositions are used in forming varistor bodies. Prior art varistors containing mostly silicon carbide are well known. More recently, metal oxide varistors have been developed which offer superior performances, particularly in power line overvoltage applications. The varistor bodies in such metal oxide varistors commonly are composed mostly of zinc oxide. Examples of such varistors are disclosed in U.S. Pat. Nos. 3,928,245 and 3,953,373.
In view of the ever increasing voltages employed on power transmission and distribution lines, there is a continuing need in the electric power industry for improved overvoltage protection devices. On high voltage lines, for example, it is necessary to employ numerous varistors in series. The varistors used must therefore have both good surge durability and linearity if they are to be economically practical.